Carbon exists in at least 8 different allotypes, one of which is a nanotube form.
The unique properties of carbon nanotubes (CNTs), more specifically, single walled carbon nanotubes (SWNT) have made them excellent candidates for applications in bio-sensing, (Wohlstadter, J. N., et al., Adv. Mat. 2003, 15, 1184), gene delivery (Pantarotto, D., et al., Angew. Chem., Int. Ed. 2004, 43, 5242), fuel cells (Li, W., et al., Carbon 2002, 40, 791) and nanofabrication (Wei, B. O., et al., Nature 2002, 416, 495).
Single walled carbon nanotubes (SWNTs) are hollow, tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. SWNTs typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm (B. I. Yakobson and R. E. Smalley, American Scientist, 1997, 324 337; Dresselhaus, et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19). SWNTs are distinguished from each other by a double index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped onto the surface of a cylinder. When n=m, the resultant tube is said to be of the “armchair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an armchair repeated n times. When m=0, the resultant tube is said to be of the “zig-zag” or (n, 0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig-zag pattern. Where n≠m and m≠0, the resulting tube has chirality and contains a helical twist to it, the extent of which is dependent upon the chiral angle.
Considerable research effort has therefore been devoted to development of methods to achieve stable suspensions of highly dispersed CNTs (Chen, J., et al., Science 1998, 282, 95; Liu, J., et al., Science 1998, 280, 1253; Wang, Y., et al., J. Am. Chem. Soc. 2006, 128, 95; Holzinger, M., et al., Angew. Chem., Int. Ed. 2001, 40, 4002; Mickelson, E. T., et al., J. Phys. Chem. B 1999, 103, 4318; Zheng, M., et al., Nature Materials 2003, 2, 338; Ortiz-Acevedo, A., et al., J. Am. Chem. Soc. 2005, 127, 9512; O'Connell, M. J., et al., Chem. Phys. Lett. 2001, 342, 265; Liu, P. European Polymer Journal 2005, 41, 2693; Zhao, B., et al., J. Am. Chem. Soc. 2005, 127, 8197; Chen, J., et al., J. Phys. Chem. B 2001, 105, 2525; O'Connell, M. J., et al., Science 2002, 297, 593 and Islam, M. F., et al., Nano Lett. 2003, 3, 269).
Progress, however, has been impeded, by two major hurdles. First, their poor solubility in both aqueous and organic solvents makes them difficult to manipulate and functionalize. Second, CNTs are generally formed as heterogeneous mixtures of metallic and semiconducting tubes with varying chiralities. In order to separate and purify the different forms of CNTs in a sample, they must first be solubilized in an appropriate medium.
Consequently, there remains a long felt need for methods to achieve simple, rapid and nondestructive solubilization of carbon nanotubes and SWNTs in particular.